The proposed research is based on my observation that steroid receptor binding proteins contain a site which both binds protease inhibitors (e.g. DFP, TPCK, PMSF, TLCK) and protease substrates (e.g. TAME, TME) and regulates steroid hormone binding. Three questions this finding raises are: (1) What is the structure of the protease inhibitor binding site on steroid hormone binding proteins? (2) How does the structure of this regulatory site compare with that on "serine" enzymes, such as chymotrypsin? (3) Is catalytic activity associated with this regulatory site? We will study these questions using two steroid hormone binding proteins, rat alpha-fetoprotein and the progesterone receptor from chick oviduct, which can be purified in sufficient quantity to electrophoretic homogeneity. Our studies include: determining the amino acid(s) (histidine, serine?) at the regulatory site which react with radioactive protease inhibitors, the amino acids adjacent to these reactive amino acids, and using sensitive radioactive substrates to test if steroid hormone binding proteins have catalytic activity. This information will be compared with that known about the structure and function in serine proteases to determine what similarities and differences in mechanism exist between a steroid hormone binding protein and proteases. We expect that our studies will be useful in understanding and regulating steroid hormone action and in understanding from another perspective the requirement for proteolytic activity in proteins.